Formation of SiC grains and the 11.3μm feature in circumstellar envelopes of carbon stars.

We propose that the so-called 11.3μm emission features observed toward carbon stars can be attributed to small spherical core-mantle type grains consisting of a SiC core and a carbon mantle or small carbon grains containing SiC as an impurity. Based on the optical constants measured from bulk α-SiC, we show that as the volume fraction of the carbon mantle increases, the peak wavelength of the absorption shifts towards longer wavelengths, the strength decreases, and the band width becomes wider; the emission peak appears at 11.3μm for a volume fraction of 0.4. This behaviour would explain a variety of SiC features observed toward infrared carbon stars. The possibility for the formation of the proposed core-mantle dust grains in circumstellar envelopes of carbon stars has been investigated by applying a theory of homogeneous nucleation and grain growth. Using a simple model for circumstellar envelopes, we demonstrate that the nucleation of SiC grains always precedes that of carbon grains when the non-LTE effect, i.e. the difference between the temperatures of gas and small clusters, is taken into account. In the case of mass loss rates |˙(M)| ≲ 1.5 × 10^[-5]M☉/yr, the SiC grains are gradually coated by carbon and show a distinctive emission feature around 11.3μm. Otherwise, the condensation of SiC and carbon simultaneously occurs after an avalanche of the formation of SiC nuclei and the emission feature is almost suppressed for |˙(M)| ≳ 1.5 × 10^[-5]M☉/yr. The radiation pressure force acting on these dust grains is sufficient to drive the mass loss of carbon stars when the volume fraction of carbon is larger than 0.1. The profile of the 11.3μm feature can be a diagnostic mean of the formation process of dust grains prevailing in circumstellar envelopes of carbon stars and well reflects the differences in the mass loss rate and the evolutionary stage.